Synthetic fibers have had a generally increasing usefulness in this century replacing natural fibers such as wool and cotton because of the plurality of special properties which can be incorporated into man-made fibers. However, synthetic fibers lack an important feature of natural fibers which is a natural curl or crimp that gives masses of a natural fiber bulkiness and feel or, as termed in the fiber industry, hand. Previous solutions to the problem of providing such a curl or crimp in synthetic fibers involves inter alia: (a) producing a conjugate fiber obtained by melt spinning polymers of different properties through a specifically shaped die face, (b) asymmetric quenching of fibers immediately after extrusion to provide a difference in microstructure in the transverse direction of the fibers, and (c) mechanically crimping the fibers, for example, in a stuffing box process.
In the asymmetric quenched fibers, the difference in microstructure in the transverse direction of the fibers is provided by the difference in the rate of cooling of opposite sides of the polymer immediately after extrusion. Furthermore, if in an attempt to render the structural difference larger, a greater amount of cooling air is used, the spinning conditions become worsened, and breakage of filaments occurs, at which point the operation becomes impossible.
With respect to mechanical crimping, the fibers produced in that way generally do not have satisfactory stability and uniformity of the crimps, and fine crimps cannot be obtained.
Now a novel process has been found which can simply and economically provide a commercially usable, homogeneous, fine denier, synthetic fiber having a substantial number of curls per unit length, which fiber is generally helical in configuration. The process can be applied to homopolymers or copolymers and is adaptable to the common commercial devices for melt spinning of fibers. Such process is a substantial improvement over present methods of imparting curl to synthetic fibers and produces a novel synthetic fiber having an axially symmetric, residual tensile force differential between its outer sheath and interior portion.